Bug 1043880 added links to treeherder to the emails sent to pushers after they make a push to try.
It's now time to adjust the wording added there to reflect that treeherder is no longer experimental & to encourage more people to give it a try over TBPL.

Created attachment 8494458[details][diff][review]
Emphasise treeherder URLs in Try emails & fix for try-comm-central
* Emphasise treeherder over TBPL, since the treeherder transition is imminent.
* s/thunderbird-try/try-comm-central/ for treeherder, now that it uses the
hg/buildbot name (bug 1035222).
* Remove unused import and fix flake8 whitespace warning.
Note: Bug 1072163 will be making the TBPL output go away, which is why I've gone for the if else assignment, since it will be much cleaner once that bug has landed.